is this Carter BBS fixable?

Trying to flatten these castings usually doesn't wind up with a good result. Before you go to that last-ditch effort, try doubling or trebling up on gaskets at that junction. Make sure to use two or three of the exact same gasket; the carb kits come with multiple variants of this gasket so you might need to buy a couple-few kits. Also oil the gaskets before install (regular clean motor oil applied with a finger) to swell them and make them conform to the gaps in the castings.